Graphitization of fibrous polyamide resinous materials



United States Patent 3,547,584 GRAPHITIZATION OF FIBROUS POLYAMIDE RESIN OUS MATERIALS Joseph G. Santangelo, Morristown, N.J., assignor to Celanese Corporation, New York, N.Y., a corporation of Delaware No Drawing. Filed Dec. 18, 1967, Ser. No. 691,174 Int. Cl. C01b 31/07, 31/04 US. Cl. 23-2091 8 Claims ABSTRACT OF THE DISCLOSURE Fibrous polyamide resinous materials are converted to graphite fibers having a configuration essentially identical to that of the polyamide starting material by thermal treatment. The fibrous graphite produced according to the invention possesses highly desirable physical properties, and is particularly suited for use as a fire resistant smokeless reinforcing medium for incorporation in a binder to form a composite article.

BACKGROUND OF INVENTION The invention relates to the graphitization of fibrous polyamide resinous materials. More particularly, the invention concerns the graphitization of fibers formed from a long-chain synthetic polymeric amide having recurring amide groups as an integral part of the main polymer chain. Such materials are commonly designated by the generic term nylon.

As is well known, graphite occurs in nature as one of the two naturally occurring forms of crystalline carbon, and is usually found as a soft black mass or as flakes disseminated in metamorphitized rock. Additionally, synthetic graphite may be manufactured from some forms of amorphous carbon, such as lampblack or petroleum coke, in an electric furnace with or without the presence of a tar or pitch. The physical configuration of a graphite article produced thereby has commonly been that of a block product. -In recent years elaborate and generally time consuming procedures for the formation of fibrous graphite have been proposed in which continuous cellulosic fibers or woven textile articles formed therefrom serve as the starting material. See, for example, United States Pat. Nos. 3,107,152 to Ford et al., 3,116,975 to Cross et al., and 3,179,605 to Ohsol. Also, generally infusible synthetic polymeric fibers, such as those formed from acrylonitrile 0r copolymers thereof have served as starting materials in processes proposed for the production of fibrous graphite. See, for example, United States Pat. No. 3,285,696 to Tsunoda. There has remained, however, an unfilled need for an efficient process to convert fibrous polyamide resinous materials to graphite fibers. Since such fibrous polyamide materials are thermoplastic, thermal treatments conducted at elevated temperatures sufficient to impart graphitization have commonly been accompanied by the melting of the material, and the concomitant destruction of its original configuration.

It is an object of the invention to provide an efiicient process for the graphitization of a fibrous polyamide resinous material.

It is another object of the invention to provide a process for the production of graphite fibers having a configuration essentially identical to that of the fibrous polyamide starting material.

It is a further object of the invention to provide a composite article comprising a reinforcing core of graphite fibers formed according to the process described herein.

It is an additional object of the invention to provide a process for the expeditious formation of graphite fibers ICC without the need for excessive expenditures for elaborate equipment utilized therein.

These and other objects, as well as the scope, nature, and utilization of the invention will be apparent from the following detailed description and appended claims.

SUMMARY OF INVENTION It has now been discovered that a process for the production of fibrous graphite comprises heating a fibrous polyamide resinous material in an oxygen-containing atmosphere at a temperature of about 200 to 350 C. to produce an infusible charred product which retains the original fibrous configuration and is capable of withstanding burning when subjected to an ordinary match flame, and subsequently subjecting the charred product to a temperature of about 2500 to 3000 C. in an inert or a reducing (i.e. non-oxidizing) atmosphere until graphitization is substantially complete to produce fibrous graphite having a configuration essentially identical to that of the polyamide resinous material.

DESCRIPTION OF PREFERRED EMBODIMENTS The fibrous polyamide resinous material utilized in the present invention may be present in a variety of configurations. For instance, such material may be in the form of filaments, staple fibers, tows, plied yarns, knits, braids, fabrics, or other fibrous assemblages. In a particularly preferred embodiment of the invention the fibrous polyamide starting material is in the form of filaments.

The resinous materials utilized in the invention are long-chain relatively high molecular weight synthetic polymeric amides having recurring amide groups as in integral part of the main polymer chain. Such fibrous nylon materials may be formed according to conventional techniques which are well known in the polymerization art, and commonly possess an average molecular weight of about 12,000 to 20,000. For instance, the resinous materials are made by the reaction of a primary or secondary diamine and either a dicarboxylic acid or an amide-forming derivative of such an acid. Illustrative examples of readily available starting materials include nylon 6T which is formed by the condensation of hexamethylenediamine and terephthalic acid, nylon 66 which is formed by the condensation of hexamethylenediamine with adipic acid, nylon 6 which is obtained by the polycondensation of caprolactam, and nylon 610 which is formed by the condensation of hexamethylenediamine with sebacic acid. The particularly preferred fibrous starting materials are formed from nylon 6T or nylon 66. It is to be understood that the invention is applicable to relatively high molecular weight polyamide fibers generally, and is not limited to the specific nylon types listed.

During the initial step of the instant process the fibrous polyamide resinous material undergoes a preoxidation treatment in an oxygen containing atmosphere, and is partially carbonized to a stabilized form so that the subsequent graphitization step of the process may be endured without the concomitant destruction of the fibrous structure of the same. The preoxidation step may be conducted by heating the material in an oxygen containing atmosphere, such as ordinary air at a temperature ranging from about 200 to about 350 C. without melting the material. The exact preoxidation temperature selected is dependent upon the resistance to distortion at elevated temperatures of the polyamide and should not exceed the melting point of the same at least during the initial portion of the preoxidation treatment. While the heating is conducted a charred product results which is capable of withstanding burning when subjected to an ordinary match flame. Volatile components commonly expelled during the preoxidation step of the process include water vapor, oxygen,

and carbon monoxide and carbon dioxide resulting from a partial combustion of the polyamide. Typically a to 50 percent reduction in the weight of the material takes place during the preoxidation step of the instant process. It is believed that a cross-linking of carbon atoms occurs during the initial step of the process to produce a charred product which is substantially completely resistant to further volatilization and further combustion.

The time required to complete the initial step of the instant process varies with the preoxidation temperature employed. For instance, if a temperature as low as about 200 C. is utilized, then heat treatment in an oxygen containing atmosphere of up to about 2 hours may be desirable. Preoxidation temperatures much below 200 C. are not recommended because of the difliculty of obtaining satisfactory stabilization within a reasonable period of time. At a temperature of about 350 C. preoxidation may be satisfactorily conducted within about 30 minutes. Preoxidation temperatures much above 350 C. are not recommended because the rate of oxidation becomes faster than the rate of cross-linking, leading to excessive losses of material. In a particularly preferred embodiment of the invention utilizing 6T nylon the preoxidation portion of the process is conducted at about 315 C. for a period of about 60 minutes. The fibrous polyamide material may be satisfactorily contained in any suitable receptacle, which will withstand the temperatures employed during the initial step of the process. For instance, the fibrous polyamide may be placed on a stainless steel boat or tray and exposed to the preoxidation treatment in a conventional tube furnace in which the required elevated temperature is produced by radiation from electrically heated walls. The preoxidation reaction can be carried out also in an autoclave by heating to the required temperature/time schedule. A continuous preoxidation treatment may be accomplished by the continuous passage of a polyamide filament through a heated chamber or calcining furnace. The fibrous structure of the polyamide material is retained throughout the preoxidation step of the process.

In the final step of the instant process the charred product is heated at a temperature of about 2500 to 3000 C. in a non-oxidizing atmosphere until graphitization is substantially complete, and any remaining elements present in the charred product other than carbon, e.g. hydrogen and oxygen, are substantially completely expelled. Additional weight losses of approximately 10 to 30 percent based on the weight of the original starting material are commonly encountered during the graphitization step. During such heating the charred product assumes a whitehot state which ultimately yields a grayish-black graphitized product. The fibrous configuration of the original polyamide material is maintained, and when subjected to X-ray diffraction the graphitic nature of the product is confirmed. Suitable inert atmospheres in which the graphitization step may be conducted include nitrogen and argon. In a preferred embodiment of the invention the graphitization is attained by contacting the charred fibrous product with the reducing flame of a conventional oxyacetylene torch, such as is commonly used for welding purposes. Suitable graphitization temperatures may likewise be produced by use of an induction furnace, arc furnace, solar furnace, low temperature plasma flame, etc. Such elevated graphitization heating may generally be completed within about seconds to about 5 minutes. 'In a particularly preferred embodiment of the invention the charred product is maintained at a temperature of about 2500 C. for at least about 3 minutes. If heating periods much below 15 seconds are utilized, then substantially complete conversion to graphite fibers i commonly not achieved. Temperatures much in excess of about 3000 C. are not recommended primarily because the well-known material handling difiiculties encountered at such extreme temperatures coupled with the possibility of subliming the graphite fibers. Also, temperatures much 4 about about 3000 C. are not easily achieved by use of conventional equipment. Suitable containers or supports for the charred product during the graphitization step of the process include graphite trays, or graphite tubes.

The graphitization of the fibrous polyamide resinous material as described in detail herein imparts enhanced physical properties to the same. The thermoplastic fibrous material is converted to high strength flexible graphite fiber having a configuration essentially identical to that of the starting material.

When incorporated in a binder or matrix, the graphite fibers of the present invention serve as a highly effective reinforcing medium. The binder selected for use with the graphite fibers is preferably also resistant to elevated temperatures and of limited flammability. Organic or inorganic binders may be utilized to bond the graphite fibers and to form a solid composite article. Illustrative examples of suitable binders include: plaster of paris, concrete, etc.; silicone resins, epoxy resins, polybenzimidazole resins, etc.; metals such as aluminum, silver, steel, tungsten, etc.; ceramics from e.g. silica, alumina, zirconia, etc. If desired, conventional pigments, coloring agents, etc. may be present in the binder together with the graphitized fibers to achieve the desired decorative or appearance qualtities. In a preferred embodiment of the invention, the binder may comprise about 5 to 75 percent by weight of the composite article. The composite articles find particular utility as high strength building panels and may be used interchangeably with conventional dry wall or wall boards. Structural aluminum sheets reinforced with graphite fibers may be formed. Other uses for composite articles formed according to the invention include: Pipes, floor tiles, reinforcing coatings, reinforced moldings, seal- 'ants, containers, high temperature insulation, turbine blades, aircraft components, helicopter blades, etc.

The following examples are given as specific illustrations of the invention. It should be understood, however, that the invention is not limited to the specific details set forth in the examples.

EXAMPLE I 6T nylon filaments formed by the condensation of hexamethylenediamine and terephthalic acid and available from the Celanese Fibers Company under the designation X-ll3 were selected as the starting material. The nylon filaments were white in color, possessed a melting point of about 375 C., and a denier per filament of 3 to 5. The filament was preoxidized and converted to a charred fibrous product having a configuration essentially identical to that of the starting material while maintained at 315 C. in an air oven for minutes. When exposed to the flame of an ordinary match, the charred product did not melt nor was it consumed.

The charred fibrous product was next continuously run through a Lepel induction furnace maintained at 2500 C. while under an atmosphere of nitrogen. The residence time for the charred product within the furnace was 5 minutes. The final fibrous product possessed a gray-black graphitic appearance, a modulus of 10 million p.s.i., and a tenacity of 100,000 p.s.i. Upon subjection to X-ray diffraction a pattern characteristic of essentially pure graphite with slight orientation was obtained. The product possessed an elongation of about one percent.

EXAMPLE II The process described in Example I was repeated with the exception that nylon 66 filaments formed by the condensation of hexamethylenediamine with adipic acid were substituted for the nylon 6T filaments of Example I. Essentially identical results were obtained.

Although the invention has been described with preferred embodiments, it is to be understood that variations and modifications may be resorted to as will be apparent to those skilled in the art. Such variations and modifications are to be considered within the purview and scope of the claims appended hereto.

I claim:

1. A process for the production of fibrous graphite comprising heating a fibrous polyamide resinous material in an oxygen-containing atmosphere at a temperature of about 200 to 350 C. to produce an infusible charred product which retains the original fibrous configuration and is capable of withstanding burning when subjected to an ordinary match flame, subsequently heating said charred product to a temperature of about 2500 to 3000" C. in a non-oxidizing atmosphere, and maintaining said product at said temperature in said atmosphere until graphitization is substantially complete to produce fibrous graphite having a configuration essentially identical to that of said polyamide resinous material.

2. A process according to claim 1 wherein said polyamide resinous material is a condensation product of hexamethylenediamine and terephthalic acid.

3. A process according to claim 1 wherein said polyamide resinous material is a condensation product of hexamethylenediamine and adipic acid.

4. A process according to claim 1 wherein said charred product is subjected to an oxyacetylene flame at a temperature of about 2500 to 3000 C. until graphitization is substantially complete. a

5. A process for the production of fibrous graphite comprising heating a fibrous resinous material formed by the condensation of hexamethylenediamine and terephthalic acid in an oxygen-containing atmosphere at a temperature of about 315 C. for about 60 minutes to produce a charred product which retains the original fibrous configuration, subsequently heating said charred product to a temperature of about 2500 C. in a non-oxidizing atmosphere, and maintaining said product at said temperature in said atmosphere for at least about 3 minutes to produce fibrous graphite having a configuration essentially identical to that of said polyamide resinous material.

6. A process according to claim 5 wherein said charred product is subjected to an oxyacetylene flame at a temperature of about 2500 C. for at least about 3 minutes.

7. A process according to claim 1 wherein said nonoxidizing atmosphere is selected from the group consisting of nitrogen and argon.

8. A process according to claim 5 wherein said nonoxidizing atmosphere is selected from the group consisting of nitrogen and argon.

References Cited UNITED STATES PATENTS 3,285,696 1l/1966 Tsunoda 23209.1 3,412,062 11/1968 Johnson et al. 26037 3,427,120 2/1969 Shindo et al 23-2091 X 3,449,077 6/1969 Stuetz 23209.1

OTHER REFERENCES Kirk-Othmer Encyclopedia of Chemical Technology, vol. 10, 1953, pp. 916924.

EDWARD J. MEROS, Primary Examiner 

